AQA GCSE Combined Science: Trilogy · 5.8

Chemical analysis

50 revision questions with answers for 5.8 Chemical analysis in AQA GCSE Combined Science: Trilogy (spec 8464). Practise with flashcards, a topic quiz, a random quiz across the course, 5 crosswords, or read every question and answer below. Not official AQA material.

All 50 questions and answers

Question 1 · Spec 5.8.1.1
What is a pure substance in chemistry?
Answer:
A single element or compound, not mixed with any other substance.
Question 2 · Spec 5.8.1.1
Why can 'pure orange juice' still be a mixture chemically?
Answer:
It contains several substances, such as water, sugars and other dissolved compounds, despite the everyday label.
Question 3 · Spec 5.8.1.1
How can melting or boiling point help identify purity?
Answer:
A pure substance has a characteristic sharp melting or boiling point, which can be compared with reference data.
Question 4 · Spec 5.8.1.1
How do impurities commonly affect melting behaviour?
Answer:
They often lower the melting temperature and broaden the melting range.
Question 5 · Spec 5.8.1.1
A solid melts over 117-123 °C instead of a sharp 122 °C. What does this suggest?
Answer:
It is likely impure, although the measurement method and uncertainty should also be checked.
Question 6 · Spec 5.8.1.2
What is a formulation?
Answer:
A mixture designed as a useful product, with components measured carefully to give required properties.
Question 7 · Spec 5.8.1.2
Give four examples of formulations.
Answer:
Medicines, fuels, cleaning agents and paints; foods, fertilisers and alloys are also examples.
Question 8 · Spec 5.8.1.2
Why are proportions important in a formulation?
Answer:
Changing the amount of a component can change properties, performance, safety or suitability for the intended use.
Question 9 · Spec 5.8.1.3
What can chromatography reveal about a sample?
Answer:
It can separate components and help identify substances by comparing their behaviour under the same conditions.
Question 10 · Spec 5.8.1.3
Name the two phases in chromatography.
Answer:
A stationary phase and a mobile phase; the solvent is the mobile phase in paper chromatography.
Question 11 · Spec 5.8.1.3
Why does the solvent move up the paper?
Answer:
It is drawn through the paper, carrying dissolved sample components with it.
Question 12 · Spec 5.8.1.3
Why do different substances travel different distances?
Answer:
They have different distributions between the mobile and stationary phases, so they move at different rates.
Question 13 · Spec 5.8.1.3
Which travels farther: a component strongly attracted to the stationary phase, or one favouring the solvent?
Answer:
The component favouring the solvent travels farther; stronger stationary-phase attraction slows movement.
Question 14 · Spec 5.8.1.3
What does more than one separated spot from a sample show?
Answer:
The sample contains more than one coloured component and is a mixture.
Question 15 · Spec 5.8.1.3 · Required practical
Why draw a chromatography baseline in pencil?
Answer:
Graphite does not dissolve and travel with the solvent as ink might.
Question 16 · Spec 5.8.1.3 · Required practical
Why is an ink baseline unsuitable?
Answer:
Its dyes could dissolve and separate, contaminating or confusing the sample results.
Question 17 · Spec 5.8.1.3 · Required practical
Why must the solvent level begin below the sample spots?
Answer:
If immersed, the samples may dissolve into the solvent reservoir instead of travelling up the paper.
Question 18 · Spec 5.8.1.3 · Required practical
Why use a small concentrated spot rather than a large wet patch?
Answer:
A small spot produces sharper separated spots and reduces overlap between components.
Question 19 · Spec 5.8.1.3 · Required practical
Why allow a sample spot to dry before adding more sample?
Answer:
It allows concentration to increase without making the initial spot spread excessively.
Question 20 · Spec 5.8.1.3 · Required practical
Why can a lid help a chromatography experiment?
Answer:
It reduces solvent evaporation and helps maintain consistent conditions inside the container.
Question 21 · Spec 5.8.1.3 · Required practical
Why mark the solvent front immediately after removing the paper?
Answer:
The solvent may evaporate quickly, making the front difficult to locate later.
Question 22 · Spec 5.8.1.3
State the Rf equation.
Answer:
Rf = distance travelled by substance / distance travelled by solvent front, both measured from the baseline.
Question 23 · Spec 5.8.1.3 · Required practical
From where should the substance's distance be measured?
Answer:
From the baseline to the centre of its spot, along the direction of solvent movement.
Question 24 · Spec 5.8.1.3
Why does Rf have no unit?
Answer:
It is a ratio of two distances measured in the same units, so the units cancel.
Question 25 · Spec 5.8.1.3
A spot travels 3.2 cm and solvent travels 8.0 cm. Calculate Rf.
Answer:
Rf = 3.2 / 8.0 = 0.40.
Question 26 · Spec 5.8.1.3
A spot has Rf 0.75 and the solvent front travels 6.0 cm. Find the spot distance.
Answer:
Distance = 0.75 × 6.0 = 4.5 cm from the baseline.
Question 27 · Spec 5.8.1.3
A spot travels 30 mm and the solvent 50 mm. Calculate Rf.
Answer:
Rf = 30 / 50 = 0.60.
Question 28 · Spec 5.8.1.3
Can a correctly measured Rf exceed 1?
Answer:
No. A dissolved component cannot travel farther than the solvent front in this method.
Question 29 · Spec 5.8.1.3
Why compare Rf values using the same solvent and conditions?
Answer:
Rf depends on the solvent and experimental conditions, so values from different conditions may not be comparable.
Question 30 · Spec 5.8.1.3
Why run known reference dyes alongside an unknown?
Answer:
Matching positions and Rf values under identical conditions provide evidence about the unknown's components.
Question 31 · Spec 5.8.1.3
Does one spot prove a sample is pure?
Answer:
No. Components may overlap or be invisible in that method; one spot is evidence consistent with purity.
Question 32 · Spec 5.8.1.3
Two unknown samples share one spot but have different extra spots. Are they identical mixtures?
Answer:
No. They may share one component, but the extra spots show different overall compositions.
Question 33 · Spec 5.8.1.3
What could cause a dye to remain at the baseline?
Answer:
It may be poorly soluble in the solvent or strongly attracted to the stationary phase.
Question 34 · Spec 5.8.1.3 · Required practical
How could overlapping spots be investigated further?
Answer:
Repeat using a different suitable solvent, which may separate substances that travelled together initially.
Question 35 · Spec 5.8.2.1
What is the test for hydrogen gas?
Answer:
A lighted splint gives a squeaky pop.
Question 36 · Spec 5.8.2.1
Why must the splint be lighted rather than merely glowing for the hydrogen test?
Answer:
The standard hydrogen test requires ignition; the characteristic result is a pop as hydrogen reacts with oxygen.
Question 37 · Spec 5.8.2.1
A metal reacts with dilute acid and the gas gives a squeaky pop. Identify the gas.
Answer:
Hydrogen.
Question 38 · Spec 5.8.2.2
What is the test for oxygen gas?
Answer:
A glowing splint relights.
Question 39 · Spec 5.8.2.2
A gas relights a glowing splint. What conclusion is supported?
Answer:
Oxygen is present.
Question 40 · Spec 5.8.2.2
Why is 'the splint burns' an insufficiently precise oxygen-test result?
Answer:
The characteristic positive observation is that a glowing splint relights.
Question 41 · Spec 5.8.2.3
What is the test for carbon dioxide?
Answer:
Bubble it through limewater; a positive result turns the limewater milky or cloudy.
Question 42 · Spec 5.8.2.3
A carbonate reacts with acid. Which gas test is appropriate?
Answer:
Bubble the gas through limewater; cloudiness supports identification as carbon dioxide.
Question 43 · Spec 5.8.2.3
Why does recording 'a colour change' poorly describe the limewater test?
Answer:
It omits the key observation: clear limewater becomes milky or cloudy.
Question 44 · Spec 5.8.2.4
What is the test for chlorine gas?
Answer:
Damp litmus paper is bleached white.
Question 45 · Spec 5.8.2.4
Why must the litmus paper be damp in the chlorine test?
Answer:
Chlorine must dissolve in the moisture for the reactions that cause the indicator change and bleaching.
Question 46 · Spec 5.8.2.4
Why should chlorine not be identified by deliberately smelling it?
Answer:
It is harmful to inhale; use the specified damp-litmus test with appropriate small-scale handling.
Question 47 · Spec 5.8.2.4
A gas bleaches damp litmus white. Which gas does this suggest?
Answer:
Chlorine.
Question 48 · Spec 5.8.2.1
How can hydrogen and oxygen be distinguished using splints?
Answer:
Hydrogen gives a squeaky pop with a lighted splint; oxygen relights a glowing splint.
Question 49 · Spec 5.8.2.3
Gas A makes limewater milky; gas B relights a glowing splint. Identify both.
Answer:
A is carbon dioxide; B is oxygen.
Question 50 · Spec 5.8.1.3 · Required practical
How should chromatography results be recorded for comparison?
Answer:
Record spot colours, distances from the baseline, solvent-front distance and calculated Rf values, with the solvent identified.

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